The progression to AIDS dementia in HIV-positive individuals appears to be markedly accelerated in opiate drug abusers. In the CNS, HIV infects microglia, and to a lesser extent astroglia, increasing the production of oxyradicals, pro-inflammatory cytokines, and the release of HIV-1 proteins such as gp120 and Tat, which can cause injury and even death in bystander neurons. Opioid drugs of abuse can synergistically increase each of the above pathophysiological effects of HIV through direct actions on glia. We have discovered that fractalkine, which is a key regulator of microglial directed toxicity to neurons, is completely protective against the accelerated HIV-1 neurotoxicity caused by morphine co-exposure. By contrast, blocking fractalkine receptors (CX3CR1) completely mimics the neurotoxic effects of combined morphine and Tat. This suggests that morphine-HIV in combination selectively attenuate fractalkine levels or signaling via CX3CR1. Studies are proposed that hypothesize that morphine induces collateral damage in HIV encephalitis by disrupting fractalkine-CX3CR1 interactions and enhancing microglial mediated killing of neurons. These studies will unambiguously determine the specific cellular origin(s), target(s), and causal glia-neuron (or vice versa) directed signaling underlying fractalkine-mediated neuroprotection following opioid HIV Tat/gp120 exposure. In addition, the expression, release, and processing of fractalkine, the expression of CX3CR1 and key regulators of fractalkine function (ADAM10, ADAM17, and cathepsin S) will be explored in neurons, microglia, and astroglia following morphine HIV-1 Tat and gp120. Lastly, the potential neuroprotective effects of fractalkine and fractalkine "sheddase" inhibitors will be assessed in a novel Tat transgenic mouse model of HIV encephalitis co-exposed to morphine. These experiments represent a high risk/high reward strategy appropriate for the R21 mechanism with the goal of understanding how fractalkine prevents toxic neuron-glial interactions in opioid drug-HIV comorbidity. PUBLIC HEALTH RELEVANCE: Emerging evidence indicates that opioid drug abuse blocks the actions of a neuroprotective factor (termed fractalkine) normally present on the surface of neurons. We discovered that when we add fractalkine to lethal concentrations of morphine and HIV proteins it completely blocked neuronal injury and death in cell culture. The proposal will examine whether opioids cause neurons to lose fractalkine and/or whether opioids cause immune cells to ignore the protective fractalkine signal, and determine whether fractalkine could be used therapeutically.